The occurrence of field-induced surface vacancies on iridium field-ion microscope specimens

Abstract

As part of a study of the field-ion imaging characteristics of some dilute iridium-based alloys, the occurrence of “vacant-site” contrast in micrographs from specimens of two purities of iridium (one nominally oxygen-free) has been investigated. Detailed small field-evaporation sequences have been performed and the resulting helium-ion micrographs analysed by a method which allows the dependence of contrast events on various parameters to be determined. On all the types of poles studied, the surface vacancy concentration was observed to be orders of magnitude above the expected bulk level characteristic of the specimen materials' thermomechanical history. The critical parameters governing the concentrations of these events are discussed and a model introduced to explain the generation of these vacant-sites which are assumed to be field-evaporation-induced surface artefacts. At worst the observed vacancy concentrations are 1–2% of the sites observed in the high-field {420} array of the fcc field-ion image, and vary with several parameters. Such observations imply important limitations on the technique of field-ion microscopy for studying certain point defect distributions.